1. Field of the Invention
The invention related to a method for the processing of medical ultrasound images of bony structures. The invention also relates to a method and a device for computer-assisted surgery utilizing said method. The invention is used in the field of apparatus for computer-assisted surgery which include a medical image acquisition system, a work station and an image display system.
2. Description of Related Art
One field of application is that of spinal or orthopedic surgery or, generally speaking, surgery performed on a bony structure. In these cases the surgical intervention may include the insertion of a linear tool, such as a screw or a pin, into the bony structure. For example, in the case of spinal surgery, being the most difficult, a first problem exists in that the operation must be carried out without damaging the nerves, the spinal cord and the vessels. Screws are generally inserted into a vertebral pedicle at a specific angle and are buried at this angle in order to be inserted into the vertebral body as far as exactly the axis of the pedicle. This operation requires a very exact localization of the axis of the pedicle. In the case of surgery without assistance from an imaging system, it is estimated that 30% of the pedicular screws are incorrectly positioned in the case of lumbar vertebrae and that until this day surgery on dorsal and cervical vertebrae is impossible for reasons of lack of precision, because these vertebrae are much smaller than the lumbar vertebrae. A second problem consists in carrying out this type of operation with precision even in the case of severe deformation of the bony structures, so a situation far from statistically deduced a priori knowledge of the shape of the bony structures involved, that is to say far from the known standard model. A third problem consists in that the bony structures do not lend themselves for the formation of images with given types of waves. It is possible to form pre-surgery X-ray images therefrom which are convenient and very exact without excessively exposing the patient, but it is difficult, if not impossible, to operate in the presence of X-rays considering the risk of excessive exposure of the patient as well as the surgeon. However, generally ultrasound is not used in conjunction with bony structures because it is known that the ultrasound waves do not penetrate bony structures and that these waves even have difficulty in traversing the ligaments which join the bones at the area of articulations. At present it is difficult to visualize an articulation, or vertebrae, by means of ultrasound images without removing the ligaments in question from the trajectory of the ultrasound beam; on the other hand, the ultrasound waves are hazardous neither to the patient nor to the surgeon and, therefore, ultrasound images may be formed during an operation.
From the article "Computer-Assisted Spinal Surgery Using Anatomy-Based Registration" by Stephane LAVALLEE, Philippe CINQUIN. et al., published in "Computer-Assisted Spine Surgery", Article No. 32, pp. 425-448, it is already known to use a pre-surgery image, acquired by digital tomography (being an X-ray imaging technique), in co-operation with a so-called "ultrasound pointer" reference system (FIG. 32-12 of the cited publication). The object is to match an ultrasound image formed in the course of the surgical intervention with the pre-surgery X-ray image so that the surgeon can carry out, in a marked zone in the ultrasound image, a surgery plan which is defined in the X-ray image and includes the positioning of tools in an appropriate location and with an appropriate orientation. In order to achieve matching of the X-ray image and the ultrasound image, diode devices are fixed on the one hand to a bony structure of the patient in order to form a reference and to a transducer of an ultrasound echograph on the other hand, thus forming a marking system. A matching algorithm is then used to make the bony structure, represented in the pre-surgery X-ray image, register with points situated on the wave front reflected by the same bony structure which is reproduced in the ultrasound image acquired during the operation.
The method which is known from the cited publication enables the acquisition of small segments of plane curves in the ultrasound image on the basis of the fact that the upper part of the thick edges represented in the ultrasound image constitutes the true edge of the bony structure because it corresponds to the front of the wave reflected by this bony edge, which arrives first from the transducer which is positioned in the upper part of the image. The known method has been tested only on samples of vertebrae outside the body of a patient. In the hypothetical case of an application for a real operation on a patient, a difficulty is encountered in that the known method is not automated. It is actually difficult to control such a delicate operation while utilizing a method which is not automatic and is not carried out in real time. The cited publication does not provide any indication which could lead to means for automation. Citation of a reference herein, or throughout this specification, is not to construed as an admission that such reference is prior art to the Applicant's invention of the invention subsequently claimed.